Wireless communication devices often require multi-band antennas for transmitting and receiving radio communication signals often called Radio Frequency (RF) signals. For example, network operators providing service on a GSM system in a 900 MHz frequency band typically used in Asia also use a DCS system in a 1800 MHz frequency band typically used in Europe. Accordingly, GSM wireless communication devices, such as cellular radio telephones, should have dual band antennas to be able to effectively communicate at least at both of these frequencies. Also, in certain countries service providers operate on 850 MHz or 1900 MHz frequency bands.
Current consumer requirements are for compact wireless communication devices that typically have an internal antenna instead of an antenna stub that is visible to the user. Small cellular telephones now require a miniaturized antenna comprising an antenna radiator structure coupled to a ground plane, the ground planes being typically formed on or in a circuit board of the telephone. The antenna must be able to cover multiple frequency bands to, for instance, accommodate the 850 MHz, 900 MHz, 1800 Mhz and 1900 Mhz bands whilst being compact.
Internal antenna radiator structures, using a radiator element in the form of a micro-strip internal patch antenna, are considered advantageous in several ways because of their compact lightweight structure, which is relatively easy to fabricate and produce with precise printed circuit techniques or metal stamping techniques capable of integration on printed circuit boards. Most known internal patch antennas tend to have a narrow bandwidth, unless a thick but low permittivity and low conductivity dielectric substrate or mount is employed. The resulting thick substrate or mount affects antenna characteristics and limits their use in many applications, particularly in handheld mobile communication devices with severe space and weight constraints.
Conventional patch antenna assemblies have natural resonant frequencies or modes for RF and microwave applications. However, there are shortcomings when using natural resonant frequencies for antenna assemblies as they are dependent upon at least the following antenna assembly factors a) the shape and dimensions of the patch; b) the shape and dimensions of the ground plane; c) the location of the feed point contact on the patch; d) the location of the ground plane contact on the patch. Once the above factors are fixed, the resonant frequencies for the antenna assembly are also fixed. It is therefore difficult to provide a compact and economic multi-band antenna assembly more specifically a quad-band antenna assembly, using a single patch antenna (or any antenna formed from a conductive sheet) for use in a radio communications device. Also, multi-band patch antennas have a common effective length for both transmit and receive modes within an operating band that can compromise antenna efficiency.
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